A heat exchanger typically comprises a plurality of parallel spaced apart fins defining air flow passages. The fins are often made of aluminum because of its excellent heat conductance and are designed to have as large a surface area as possible in order to increase the heat radiation and cooling effect. The spacing between adjacent fins is narrow in order to provide more cooling surface area in as small a space as possible. As a result, particularly when the exchanger is used for cooling, moisture from warm air passing through the exchanger condenses on the cold fin surfaces and, if those surfaces are hydrophobic, beads of water accumulate and can block the narrow passages between fins. This phenomenon is called bridging. The increased resistance to air flow caused by bridging reduces the efficiency of the heat exchanger. In order to inhibit the accumulation of water between the fins, the surfaces of the fins are preferably made hydrophilic so that a thin film of water coating the surface can slide off quickly, a phenomenon known as sheeting off.
Various methods exist for making such surfaces hydrophilic. These methods include coating the surface with acrylic acid resin alone or with a mixture of the resin and sodium silicate; or treating an aluminum surface with a continuous film of a water soluble basic polymer having colloidal aluminum dispersed therein and curing the polymer to water insolubility followed by hydrolyzing the surface of the cured polymer film. See U.S. Pat. No. 4,181,773. In U.S. Pat. No. 4,503,907 a mixture of water soluble acrylic resin and a water soluble amino resin is mixed with a synthetic silica and a surface active agent such as a polyoxyethylene glycol and the mixture is applied to a fin surface and baked. U.S. Pat. No. 4,588,025 discloses a fin treatment containing an alkali silicate and a carbonyl-containing low molecular weight organic compound such as an aldehyde, ester or amide. U.S. Pat. No. 5,012,862 discloses a coating of polysulfonic acid on aluminum heat exchanger fins to render the surface hydrophilic. U.S. Pat. No. 5,342,871 discloses a combination of an ethylene/acrylic acid copolymer and a water soluble amine salt of a fatty acid as providing a hydrophilic coating for heat exchanger fin surfaces.
The majority of prior art methods for imparting a hydrophilic surface to aluminum surfaces involve applying water soluble polymers. While such methods generally provide good hydrophilicity when freshly applied, the hydrophilicity often decreases after exposure to air and/or water. The long retention of hydrophilicity on the aluminum surfaces of fin type heat exchangers is critical to performance.